The enhancement of EM fields in the vicinity of metallic nanoparticles and metallic nanostructures can be explained by the phenomenon of localized surface plasmon resonance. The shape and magnitude of associated features measured in the transmission or reflection spectra from these metallic structures depend on the enhanced scattering and absorption of light at specific wavelengths. The details of the extinction cross-section enhancement over a finite wavelength range is affected by several different factors that include the characteristic optical constants and geometry of the nanostructures illuminated by incident light as well as the optical constant of the surrounding matrix phase.
The origin of plasmon resonances are collective oscillations of the conduction band electrons, and they result from the presence of interfaces for nanoparticles and films of a select group of materials which include the noble metals Ag, Cu, and Au as well as the so-called transparent conducting metal oxides that include Sn-doped In2O3 (ITO), Al-doped ZnO (AZO), and Nb-doped TiO2 (NTO). Localized surface plasmons are excited when light is incident on metallic nanoparticles which typically have dimensions smaller than the wavelength of the incident light. At certain characteristic wavelengths, one or more resonant modes are excited in the nanoparticles, leading to a significant enhancement in absorbed and scattered light and a strong increase in the electromagnetic fields in the vicinity of the particles. Localized surface plasmons can be detected as resonance peaks in the absorption and scattering spectra of the metallic nanoparticles. Nanostructures made up of noble metals, such as gold, silver, and copper, and the more recently discovered conducting metal oxides such as ITO, AZO, and NTO are well known to exhibit localized surface plasmon resonance (LSPR) phenomena.
The collective oscillation of the free electrons is also sensitive to changes in the size and shape of the particle. For example, gold nanoparticles embedded in a transparent matrix phase with a real dielectric constant similar to that of SiO2 (ε˜2.25) and average diameters in the range of approximately 5-10 nm, strongly absorb at visible wavelengths with a maximum absorbance near 520 nm. In this particular case, the energy required to excite the surface plasmon lies in the visible region of the spectrum but the surface plasmon resonance energy can also be tuned to occur at wavelengths ranging from ultraviolet to near-infrared. With increases in the Au particle size, a shift in the peak of the optical absorption to longer wavelengths is observed due to the excitation of higher-order resonant modes. The relative magnitude of the scattering cross-section also increases as compared to the absorption cross-section resulting in particles that strongly scatter light rather than absorb it for particle sizes approaching 100 nm. The plasmon resonance band is also sensitive to particle shape. For example, an elliptical or rod-shaped particle can exhibit two characteristic plasmon resonance bands at distinct wavelengths depending upon the orientation of the particle with respect to the polarization state of incident electromagnetic radiation. In addition to being size- and shape-dependent, the plasmon resonance band is sensitive to changes in the dielectric properties of the surrounding medium. For transparent matrix media with large dielectric constants, the energy required to collectively excite the electrons is decreased thereby shifting the peak in the extinction cross-section to lower energies and longer wavelengths.
The strong dependence of the optical extinction peak on a number of material dependent parameters provides the nanoparticles with an inherent sensing ability. For visible light, generally only changes in refractive index occurring at distances within about 200 nm of the particle surface result in changes to the optical properties of the nanoparticles. The plasmon resonance behavior of nanoparticles are particularly sensitive to adsorption directly on the particle surface and hence biological sensing based on analyte absorption by nanoparticles and subsequent modifications of the absorbance maximum is currently an area of significant effort.
Changes in the absorbance maxima generated by the localized surface plasmon resonance effect have also been utilized extensively for gas sensing applications in the low and intermediate temperature ranges. A select few researchers in the field have also applied Au incorporated films to optical gas sensing at higher temperatures. Current technical literature suggests that the gas sensing response of technically useful noble metal/metal oxide composite films depend significantly upon the selection of a matrix phase that can play an active role in the gas sensing process. Two potential ways that such an active role can be played include (1) a change in the free carrier density of the matrix phase followed by an electronic charge transfer from the matrix to the nanoparticle and (2) a change in the effective dielectric constant of the matrix phase. Both of these effects would result in a modification to the extinction peak of noble metal nanoparticles associated with the localized surface plasmon resonance effect that could be detected through optical based monitoring techniques. However, direct interactions between the metallic nanoparticles and chemical constituents of the ambient gas atmosphere may also be important under certain testing conditions. For example, direct adsorption of chemical species on Au nanoparticles resulting in electronic charge transfer may impart significant modifications to the extinction peak of Au-nanoparticles in the case of Au-nanoparticle incorporated inert oxides such as SiO2 and Al2O3 where interactions between the matrix and the ambient atmosphere are significantly reduced as compared to catalytically active and reducible oxides such as TiO2, YSZ, and ZnO. Such direct interactions are expected to be particularly important in the case of conducting metal oxide nanoparticle incorporated films as relatively large changes in free carrier density of the conducting metallic oxides in response to changes in the chemical composition of ambient gas atmospheres at elevated temperatures have been observed.
For high temperature (T>˜500° C.) optical gas sensing, Au nanoparticles have been embedded in catalytically active, reducible, and oxygen ion conducting matrices such as TiO2 or yttria-stabilized zirconium (YSZ). Dielectric matrix phases with a relatively wide bandgap and low oxygen ion and electronic conductivity commonly referred to in the literature as “inert” such as SiO2, Al2O3, and Si3N4 are expected to exhibit improved temperature and chemical stability. Recent work has therefore also employed Au nanoparticles embedded in an inert oxide matrix such as SiO2 and Al2O3 for gas sensing at temperatures as high as approximately 900° C. For lower temperatures, gold nanoparticles have been embedded in matrices of even more highly reducible oxides such as NiO, WO3, or CuO. See e.g., Sirinakis et al., “Development and Characterization of Au-YSZ Surface Plasmon Resonance Based Sensing Materials: High Temperature Detection of CO,” J. Phys. Chem. B 110 (2006); and see Ohodnicki et al., “Plasmonic Nanocomposite Thin Film Enabled Fiber Optic Sensors for Simultaneous Gas and Temperature Sensing at Extreme Temperatures”, Nanoscale 5 (19) (2013); and see Ando et al., “Optical CO sensitivity of Au-CuO composite film by use of the plasmon absorption change,” Sensors and Actuators B 96 (2003); and see U.S. Pat. No. 7,864,322 B2 to Carpenter et al.; and see U.S. Pat. No. 8,411,275 B1 to Ohodnicki et al.
In addition to noble metals that are commonly known to exhibit a surface plasmon resonance, oxides with a relatively high electronic conductivity have also been demonstrated to exhibit pronounced surface plasmon resonances. These material systems have also been demonstrated to display enhanced high temperature optical gas sensing responses as compared to corresponding oxide systems with a relatively low electronic conductivity and free carrier concentration. See e.g. Ohodnicki et al., “Plasmonic transparent conducting metal oxide nanoparticles and nanoparticle films for optical sensing applications”, Thin Solid Films 539 (2013); also see U.S. Pat. No. 8,638,440 to Ohodnicki et al. Optical sensors based upon this class of materials may show improved stability under high temperature and/or chemically aggressive harsh environment conditions in some cases due to a relatively high melting point as compared to the noble metals Ag and Au and a potential for improved corrosion resistance under certain conditions.
The temperature of a gas stream is another parameter that is important to measure in addition to the chemical composition and many different types of temperature sensors have been demonstrated and are used in practice for such applications. For example, thermocouples and resistive temperature detectors are traditional temperature sensing devices that are widely employed in which temperature can be monitored by an electrical signal that consists of a current or voltage. In many cases, it would be advantageous to monitor temperature through optical means without the need for electrical wires or signals at the location where the temperature is being monitored. For example, the use of electrical wires and signals presents a potential safety hazard in the case of flammable gas atmospheres. As another example, application of electrical based sensors in extreme high temperature environments greater than approximately 500° C. can require expensive insulation, electrical wiring, and interconnections which are also a common source of sensor failure under high temperature conditions. To overcome the disadvantages and limitations of such traditional temperature sensing approaches, a number of optical based temperature sensor methodologies have been developed. Methodologies that are compatible with optical waveguide based sensing platforms such as optical fibers for remote and distributed sensing capability are particularly advantageous. One approach to optical temperature sensing that has been explored by prior investigators consists of the development of sensors that enable monitoring of the optical transmittance or reflectance of metallic or dielectric materials. For example, an optical temperature sensor based upon measuring the temperature dependent reflected signal from an end-coated and cleaved optical fiber of a monolithic film of a dielectric material such as TiO2 with a temperature dependent refractive index has been reported. Optical temperature sensors based upon a temperature dependent band-gap of a dielectric material such as ZnO have also been reported. See e.g., M. Naci Inci et al., “A fibre-optic thermometric sensor based on the thermo-optic effect of titanium dioxide coatings”, Optics & Laser Technology 29 (3) (1997); and see e.g. S. Chenghua et al., “Optical temperature sensor based on ZnO thin film's temperature dependent optical properties”, Rev. Sci. Instrum. 82 (2011). Temperature sensors that are based upon monitoring the optical reflectance of continuous thin films of a metal such as Au or Ag which are well known to display a surface plasmon resonance have also been proposed and are advantageous due to a relatively high theoretical sensitivity of optical transmittance or reflectance to temperature. See e.g. S. Ozdemir et al., “Temperature Effects on Surface Plasmon Resonance: Design Considerations for an Optical Temperature Sensor”, Journal of Lightwave Technology 21 (3) (2003); and see e.g. A. Sharma et al., “Theoretical model of a fiber optic remote sensor based on surface plasmon resonance for temperature detection”, Optical Fiber Technology 12 (2006). Such approaches can be effective for monitoring of temperature under certain conditions, but it would be advantageous to provide an alternative optical-based methodology that employs temperature sensing materials which exhibit improved temperature stability, chemical stability, and/or additional functionality as compared to continuous films of a dielectric material such as ZnO or TiO2 or continuous films of a metal displaying a surface plasmon resonance such as Ag or Au. It would be further advantageous if the optical methodology were relevant for monitoring of temperature over a broad range of temperatures spanning from cryogenic to extreme, high temperature conditions greater than approximately 500° C. by monitoring the optical properties of the nanocomposite.
Such advantages can be provided by metal nanoparticle incorporated dielectric nanocomposite materials of the type extensively investigated for elevated temperature chemical sensing applications such as Au/TiO2, Au/YSZ, and Au/SiO2. An optical temperature sensor employing a subset of such materials systems has been discussed theoretically by previous authors, but in that case the temperature sensing mechanism involved a particle size that varied with temperature due to coarsening. Such temperature dependent particle coarsening would impart a time-dependent optical signal at fixed temperature and an irreversible temperature response with increasing temperature that would prevent a useful and reversible temperature sensing response. See e.g. S. K. Srivistava et al., “Simulation of a localized surface plasmon resonance based fiber optic temperature sensor”, Journal of the Optical Society of America 27 (7) (2010). It would instead be advantageous to employ metal nanoparticle incorporated sensor materials for which the particle size does not vary significantly under the testing conditions employed. Stabilization of the particle size can be achieved, for example, through dielectric matrix selection (thickness, composition, etc.) and high temperature pre-treatments of the nanocomposite temperature sensor material. See e.g. Ohodnicki et al., “Plasmonic Nanocomposite Thin Film Enabled Fiber Optic Sensors for Simultaneous Gas and Temperature Sensing at Extreme Temperatures”, Nanoscale 5 (19) (2013); also see Ohodnicki et al., “Plasmon Resonance at Extreme Temperatures in Sputtered Au Nanoparticle Incorporated TiO2 Films”, Proceedings of SPIE Optics and Photonics 8456 (2012); also see Buric et al., “Theoretical and experimental investigation of evanescent wave absorption sensors for extreme temperature applications”, Proceedings of SPIE Optics and Photonics 8816 (2013); also see Buric et al., “Optical fiber evanescent absorption sensors for high-temperature gas sensing in advanced coal-fired power plants”, Proceedings of SPIE Optics and Photonics 8463 (2012).
It would be further advantageous if an optical interrogation methodology allowed for multiple parameters to be monitored simultaneously through multiple wavelength interrogation in these and related materials by exploiting the detailed wavelength dependence of optical properties of a nanocomposite material. For example, it would be advantageous if such an optical interrogation methodology would allow for simultaneous monitoring of the temperature and chemical composition of a gas stream or a liquid phase.
Under most conditions, nanoparticles of metals such as Pd, Pt, Rh, Ru, Os, and Ir do not display a pronounced absorption or scattering localized surface plasmon resonance. As such, nanocomposite films consisting of nanoparticles of these metals dispersed in a dielectric matrix phase have not been extensively investigated for optical sensing applications. However, many of these metals exhibit relatively high melting points as compared to the noble metals Au and Ag for which localized surface plasmon resonances are well known to occur. It would therefore be advantageous to develop materials with useful high temperature optical sensing responses based upon this class of materials, particularly for applications in the most demanding temperature ranges where typical noble metals such as Ag and Au may undergo melting
Disclosed here is a method for temperature sensing utilizing optical signal shifts that are associated with temperature dependent optical properties of a metal nanoparticle incorporated nanocomposite material. The temperature sensing material is comprised of a plurality of metallic nanoparticles dispersed in a dielectric matrix. The method disclosed offers significant advantage over materials typically utilized for temperature sensing, including enhanced thermal and chemical stability of the dielectric matrices due to embedded metallic nanoparticles, increased chemical and size stability of metallic nanoparticles embedded in a dielectric matrix, and the capability for simultaneous monitoring of temperature and chemical composition of a gas stream or a liquid through multiple wavelength interrogation, among other advantages. In addition, several candidates for dielectric matrix materials (e.g. SiO2, Al2O3, MgF2 doped SiO2, mixed SiO2/Al2O3) exhibit relatively low values of refractive indices for fully densified films ranging from less than ˜1.5 to greater than ˜1.7. This property is advantageous as it enables integration of nanocomposite films directly with optical fiber based sensors as a gas sensitive cladding layer in an evanescent wave absorption spectroscopy based sensing configuration, while maintaining the conditions necessary for waveguiding in the low refractive index core material. These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.